In recent years, it is proposed to cyclically change a PWM signal using a spectrum spreading technique, when a switching power supply device applies the PWM signal as its control signal. In this conventional power supply device, a switching frequency control unit generates a spectrum spreading signal whose period changes according to a clock signal. An output voltage detection unit detects an output voltage. A reference voltage unit generates a reference voltage. A difference calculation unit calculates a difference between the output voltage and the reference voltage. A compensator generates a compensation value based on an input voltage of the switching power supply unit, the reference voltage and the period of the spectrum spreading signal. A PWM generator generates a PWM signal based on the spectrum spreading signal and the compensation value. The reference voltage unit varies the reference voltage in accordance with a change in the period of the spectrum spreading signal. This conventional power supply device executes spectrum spreading processing by using a system clock.